1. Field of the Invention
The present invention relates to an asymmetric xcex2-ketoiminate ligand compound. More particularly, the present invention relates to an asymmetric xcex2-ketoiminate ligand compound, which is excellent in terms of chemical stability, thermal properties, and solubility in organic solvents. Also, the present invention is concerned with a method for preparing the ligand compound and with an organometallic precursor containing the ligand compound, suitable for use in chemical vapor deposition.
2. Description of the Prior Art
For use as dielectric thin films, ferroelectric thin films, electrodes, etc., metal oxide films are formed by various deposition methods which are representatively exemplified by sputtering technique, sol-gel technique, and chemical vapor deposition technique. Using volatile organometallic compounds as sources for metal oxide films, chemical vapor deposition methods have attracted keen attention owing to their advantages of high deposition rate, deposition uniformity over large areas, excellent step coverage, and easy control of metal composition ratio (See: Adv. Materials for Optics and Electronics, 1993, 2, 271).
Use of excellent precursors meeting certain standards is a prerequisite for success in the preparation of thin films through chemical vapor deposition. Useful precursors are required to exhibit outstanding thermal and chemical stability, excellent evaporation or sublimation properties, low chemical toxicity, large difference between evaporation and decomposition temperatures, and no reactivity with other compounds in a reactor during evaporation or delivery in gas phases. Additionally, in order to obtain a multi-component thin film of high quality, composition ratio of individual metal components introduced into the thin film must be easy to control and their respective precursors must be similar to one another in decomposition behavior at a deposition temperature.
Some recent studies into precursors for use in chemical vapor deposition have been performed with M(xcex2-diketonate)n precursors using xcex2-diketone as a ligand and with M(OR)x (xcex2-diketonate)y precursors using alcohol and xcex2-diketone as co-ligands. However, these precursors are discovered to have drawbacks in aspects of volatility, formation of residues after evaporation, instability to moistures, and reactivity with other precursors. When preparing multi-component metal oxide thin films, such as BST (barium strontium titanate), by use of such precursors, the great difference between the volatilization properties of a titanium precursor and of barium and strontium precursors makes the chemical vapor deposition difficult.
There have been reported precursors using xcex2-ketoimine ligands in which one of the oxygen atoms of xcex2-diketone is substituted with a nitrogen atom. For example, precursors represented by Mn+(xcex2-ketoiminate) n are disclosed in U.S. Pat. No. 4,950,790. Employing xcex2-ketoiminate as a ligand, these precursors are improved in thermal and chemical stability, but poor in resistance to hydrolysis.
Japanese Pat. Laid-Open Publication No. Hei. 6-298,714 introduces the use of N-alkoxy-xcex2-ketoimine as a ligand, teaching that metal complexes can be prepared by reacting a symmetric ligand having a group, such as xe2x80x94CF3 or t-butyl, substituted at both ends of xcex2-ketoimine, or an asymmetric ligand of xe2x88x921 equivalent having a substituent only at a carbon of the ketone half with +2 equivalent metal such as Ba, Ca and so forth. The N-alkoxy-xcex2-ketoimine can be obtained by an ordinary preparation method for xcex2-ketoimines in which substituted xcex2-diketone is reacted with amine alcohol in the presence of an acid catalyst.
For the convenience of chemical vapor deposition, precursors are generally delivered in liquid phases. Because this liquid delivery chemical vapor deposition commonly employs n-butyl acetate as a solvent, the solubility of precursors for liquid delivery chemical vapor deposition in the solvent is of particular importance. A precursor which has a low solubility in n-butyl acetate may cause the problem of clogging the delivery pipe only after several uses thereof in the preparation of thin films. This problem is also caused upon using N-alkoxy-xcex2-ketoiminate type ligands in the preparation of the precursor, although they improve the stability of the precursor.
Therefore, there remains a need for a ligand that can be used to produce a precursor which is highly soluble in such a solvent in addition to exhibiting excellent thermal and chemical stability, excellent evaporation or sublimation properties, low chemical toxicity, large difference between evaporation and decomposition temperatures, and no reactivity with other compounds during evaporation or delivery in gas phases.
Leading to the present invention, the intensive and thorough research into organometallic precursors for chemical vapor deposition resulted in the finding that modification of N-alkoxy-xcex2-ketominate ligands in such a way as for the imine half of the xcex2-kotomine backbone to bear more carbon atoms than the ketone half produces novel aymmetric ligands which can be used to produce precursors highly soluble in general organic solvents including n-butyl acetate.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a xcex2-ketoiminate ligand which is greatly increased in solubility, as well as showing excellent evaporation properties and thermal and chemical stability.
In accordance with an aspect of the present invention, there is provided an asymmetric xcex2-ketoiminate ligand compound represented by the following chemical formula (4): 
wherein, R1 is a linear or branched alkyl group containing 1 to 8 carbon atoms; R2 is a linear or branched alkyl group containing 2 to 9 carbon atoms, with the proviso that R2 contains more carbon atoms than R1; Rxe2x80x2 is a linear or branched alkylene group containing 1 to 8 carbon atoms or a hydrocarbon containing 1 to 3 ethylene ether or propylene ether moieties, represented by xe2x80x94(CH2)nOxe2x80x94 (n=2 or 3); R3 is a hydrogen atom or a linear or branched alkyl group containing 1 to 9 carbon atoms; and X is an oxygen atom or a sulfur atom.
In accordance with another aspect of the present invention, there is provided a method for preparing the asymmetric xcex2-ketoiminate ligand compound, comprising the step of reacting an xcex1,xcex2-yne-one compound, represented by the following chemical formula (1) with a primary amine represented by the following chemical formula (2) or with a secondary amine represented by the following chemical formula (3): 
R3Xxe2x80x94Rxe2x80x2xe2x80x94NH2xe2x80x83xe2x80x83(2)
xe2x80x83(R3Xxe2x80x94Rxe2x80x2)2xe2x80x94NHxe2x80x83xe2x80x83(3)
wherein, R1 is a linear or branched alkyl group containing 1 to 8 carbon atoms; and R2 is a linear or branched alkyl group containing 2 to 9 carbon atoms, with the proviso that R2 contains more carbon atoms than R1; Rxe2x80x2 is a linear or branched alkylene group containing 1 to 8 carbon atoms or a hydrocarbon containing 1 to 3 ethylene ether or propylene ether moieties, represented by xe2x80x94(CH2)nOxe2x80x94 (n=2 or 3); R3 is a hydrogen atom or a linear or branched alkyl group containing 1 to 9 carbon atoms; and X is an oxygen atom or a sulfur atom.